1. Field of the Invention
The present invention relates to a process for electrostatically pinning a dielectric film to a moving surface which is electrically conducting and connected to a fixed potential, the film being applied to the moving surface with the aid of a corona electrode consisting of a metal wire fed with a direct current, the electrode being arranged parallel to the said surface, and a second uninsulated electrode being arranged near and parallel to the corona electrode and perpendicular to the direction of forward movement of the dielectric film.
2. Description of the Prior Art
The process for electrostatically pinning a dielectric film is described in U.S. Pat. No. 3223757. The corona electrode is subjected to a high voltage causing the ionization of the air in the region around the wire. The ions of the same sign as that of the voltage applied to the wire are strongly attracted towards the grounded cooling surface and deposit on the film, which causes the latter to be attracted and applied to the cooling surface. However, it was observed that the speed of application of the molten web to the cooling surface is restricted by the appearance of microbubbles of air between the web and the cooling surface, which leads to a heterogeneity in the appearance of the film. An attempt was then made to overcome this disadvantage by increasing the voltage of the corona wire, but the appearance of electric arcs, as soon as the breakdown voltage of the dielectric situated between the corona electrode and the cooling surface is reached, rapidly restricts this voltage increase.
It was then proposed, in U.S. Pat. No. 3,427,686, to bring the cooling surface to a potential between the value of the corona electrode potential and ground potential, so as to maintain a sufficiently high field between the electrode and the drum, allowing the attraction of the ions emitted by the corona electrode. The risk of disruptive discharge between the electrode and the drum is thus avoided, without substantially increasing the speed of application.
More recently, it was proposed, in U.S. Pat. No. 3,655,307, to provide the corona electrode with a grounded metal counter-electrode being covered with an insulating layer which prevents the formation of electrical discharges between the two electrodes and made it possible to apply an exceptionally high voltage to the corona wire without interruption of the application force due to disruptive discharges. The ions attracted towards the counter-electrode by ground potential retain their charges and deposit on the insulating surface which thus becomes positively charged and then repels the positive ions generated by the corona electrode; this produces the effect of focusing the ions towards the molten web. The intensity of the corona current is increased by injecting a gas in the region of the corona electrode. An improvement in the speed of application of the molten web to the cooling surface is thus achieved. However, a device of this kind exhibits disadvantages: the voltage to which the corona electrode is subjected is high and the counter-electrode must be protected by a strong dielectric material in order to avoid electric arcs between the electrode and the counter-electrode. Furthermore, in order to prevent the formation of an electric arc between the cooling drum and the corona electrode, without increasing the distance between the two, it is necessary to inject a gas of high dielectric strength in the region of the corona electrode. Because of such constraints, a device of this kind is rather difficult to produce and is relatively expensive.
Another simpler solution was proposed in U.S. Pat. No. 3,820,929, which describes a device comprising a corona electrode and a second electrode, arranged near the corona electrode, these two electrodes being subjected to the same potential, one of them having a small diameter and ionizing the air which surrounds it, and the second being of larger diameter and having no ionizing action on the ambient medium. Since this second electrode is at a positive potential, it strongly repels the positive ions towards the grounded cooling drum, causing these ions to focus towards the molten mass, which improves the application of the molten mass to the drum.
However, a device of this kind presents dangers from the point of view of safety in industrial use, because of the high voltage present on this counter-electrode. Furthermore, the improvement in the performance is small as compared to the device having a single wire.